| dc.description.abstract | The Chinshui gas field located in the fold-thrust belt of western Taiwan was a depleted reservoir. Recently, CO2 sequestration has been planned at shallower depths of this structure. CO2 injection into reservoir will generate high fluid pressure and trigger slip on reservoir-bounding faults. We present detailed in-situ stresses from deep wells in the Chinshui gas field and evaluated the risk of fault reactivation for underground CO2 injection. The magnitudes of vertical stress (Sv), formation pore pressure (Pf) and minimum horizontal stress (Shmin) were obtained from formation density logs, repeat formation tests, sonic logs, mud weight, and hydraulic fracturing including leak-off tests and hydraulic fracturing. The magnitude of maximum horizontal stress (SHmax) was constrained by frictional limit of critically stressed faults. Results show that vertical stress gradient is about 23.02 MPa/km (1.02 psi/ft), and minimum horizontal stress gradient is 18.05 MPa/km (0.80 psi/ft) or equivalent to 0.8 of Sv. Formation pore pressures were hydrostatic at depths 2 km, and belowe depths 2 km, formation pore pressure increase with a gradient of 19.47 MPa/km (0.86 psi/ft) in the middle block and 16.62 MPa/km (0.73 psi/ft) in the south block. The ratio of fluid pressure and overburden pressure (λp) below depth 2 km is 0.59. Lower than normal pressures (average 12.95 MPa/km) are observed in the gas-bearing reservoir of the Talu-sand. The upper bound of maximum horizontal stress constrained by strike-slip fault stress regime and coefficient of friction (μ=0.6) is about 15.07 MPa/km (0.67 psi/ft) in the middle block and 18.55 MPa/km (0.82 psi/ft) in the south block. The orientation of maximum horizontal stresses was calculated from four-arm caliper tools through the methodology suggested by World Stress Map (WMS). The mean azimuth of preferred orientation of borehole breakouts are in about N80。E. Consequently, the maximum horizontal stress axis trends in N170。E and sub-parallel to the far-field plate-convergence direction.
Geomechanical analyses of the reactivation of pre-existing faults was assessed using 3DStress and Traptester software. Under current in-situ stress, the F2 fault in middle block has higher slip tendency, but still less than frictional coefficient of 0.6 a common threshold value for motion on incohesive faults. At the depth of the TL-sand, approximately 14.5 MPa of excess pressure would be required to cause the F2 fault to slip. Sensitivity analysis of the parameters affecting the slip potential indicates that Shmin has highest effect on the faults stability. Scenarios tests indicate that 8.85 MPa excess pore pressure would be required to cause the optimal oriented F2 fault to reactivate. This corresponds to CO2 column heights of 1290 m, whereas the height of structural closure of the TL-sand does not exceed 750 m. The results also indicate that CO2 injection in the Chinshui gas field will not compromise the stability of faults.
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